A LAN (Local Area Network) standardization work has been promoted in the IEEE (Institute of Electrical and Electronics Engineers) 802 committee and the IEEE 802.11 subcommittee is carrying out a study on a wireless LAN system.
For an access control method between a plurality of client terminals CL in a wireless LAN system, a Carrier Sense Multiple Access with Collision Avoidance+Acknowledge mode (CSMA/CA+Ack) is employed as a MAC layer protocol, which is standardized by IEEE 802.11.
In the carrier sense multiple access with collision avoidance+acknowledge mode, communication is started after a client terminal wishing to communicate checks whether a carrier wave is sensed or not to confirm that communication has not been already performed by other client terminals.
FIG. 1 is a time chart showing how communication is performed in accordance with the carrier sense multiple access with collision avoidance+acknowledge mode. A plurality of client terminals CL1 to CL3 communicates through an access point AP.
In FIG. 1, for example, while the client terminal CL1 performs data communication, other client terminals CL2, CL3 cannot transmit data since a carrier wave is sensed during the communication of the client terminal CL1.
Other client terminals CL2, CL3 can transmit data after the carrier wave is not sensed because of termination of the data transmission of the client terminal C11.
However, in such a system, if the plurality of client terminals CL1 to CL3 is distant from each other such that radio waves transmitted by other client terminals cannot be sensed, a plurality of client terminals CL may determine that data can be transmitted and a collision may be generated by performing data transmission.
To avoid such inconvenience, a band reservation request mode (RTS/CTS: request to send/clear to send) is prescribed in IEEE 802.11 as another mode. FIG. 2 is a time chart how communication is performed in accordance with the band reservation request mode (RTS/CTS: request to send/clear to send).
In this mode, for example, a client terminal CL2 requesting communication transmits a band reservation request (RTS) frame. When receiving this RTS frame, an access point AP returns a transmission confirmation (CTS) frame to provide the client terminal CL 2 with the transmission right.
On this occasion, a MAC address of the client terminal CL provided with the transmission right is described in a destination MAC address region of the CTS frame and time (duration) required until data transmission/Ack frame response is described as well. Since electric waves can be received by all the client terminals CL within a reachable range from the access point AP, the CTS frame is received by all the client terminals CL.
Therefore, all the client terminals CL can determine which client terminal CL obtains the transmission right. The client terminals CL without the transmission right halts operation during the time described in the CTS frame. In this way, radio waves transmitted by a plurality of client terminals CL can be prevented from colliding.
However, in either access control mode shown in FIG. 1 and FIG. 2 (CSMA/CA, RTS/CTS) described above, only one wavelength band is used. Therefore, if another client terminal transmits radio waves, transmission cannot be performed. The access control is performed on the “first-come-first-served” basis between a plurality of client terminals CL when radio waves can be transmitted. Therefore, quality of service (QoS) cannot be guaranteed.
On the other hand, as a known technology, an example using a plurality of wavelength bands is proposed (patent document 1). In the technology described in the patent document 1, a different communication band is assigned to each of stations (terminals) corresponding to a plurality of client terminals. As a feature, when another terminal submit a communication request for one terminal already in communication, a control station puts the communication request into a queue and the communication is enabled in the order of the queue when the preceding communication is terminated.
Therefore, the technology described in patent document 1 is different from a technology on the premise of the carrier sense multiple access with collision avoidance (CSMA/CA) mode that senses the presence of the carrier wave to control the access.
(Patent Document 1)
Japanese Patent Application Laid-Open Publication No. 08-186567